Sudden Drop in (Video) Performance

I have an old (2006) iMac, which for a long time I have been using as a media centre / TV. Recently I have been having lots of problems when playing video (both streaming and from hard drive). The main symptoms are:

Video momentarily looping a couple of frames, while audio continues.
Video stopping completely while audio continues - computer becomes unresponsive.
Bits of graphics from closed windows remaining on screen after they have been closed.

Anyone able to suggest what the problem is, or even better how to fix it?

It sounds like the video card is flaking out. Might be age, or dust.

Another possibility is that the hard drive is starting to fail, or just needs a serious defragging.

At eight years old, I'd have said you got your use out of it and its time to buy a more recent refurb'd Mac mini to replace it (also would be way better at handling HD video).

Thanks for the suggestions chas_m

I was under the impression that Macs don't benefit from defragging. If that's not the case, I guess it's worth a try. Would you recommend any software to do this?

I'll be sad to see the iMac go, if it has had its day, I'm quite fond of it. Its my only remaining Mac that (in my mind) looks like an Apple product.